1. Field of the Invention
The invention relates to programmable controllers for automatically controlling electronic devices, and more particularly to a programmable controller capable of variably setting a number of input/output signals for the electronic devices.
2. Discussion of the Related Art
A block-type programmable controller, which is typical of conventional programmable controllers, has all of its electric circuits contained within a housing. This includes the input/output (I/O) elements required by the controller to convert received information signals into a form suitable for information storage and processing. If a particular I/O element experiences problems or fails completely, replacement of the I/O element is difficult given the enclosed structure of the circuits. Replacement usually involves replacing the whole unit or swapping a card within the unit even if only a single I/O element is defective. Furthermore, the manufacturer is generally the only party able to replace I/O elements.
To overcome such shortcomings, a block-type programmable controller, as shown in FIG. 19, with a replaceable output element is known in the art. In FIG. 19, reference numerals 51, 52 designate a control unit and a power unit contained in a case; 53, an input unit for receiving information signals from an electronic device under control; 54, an output unit for sending information signals to the controlled electronic device; and 57, a replaceable output element for level converting output data signals. The output element 57 is replaceable via a connecting socket (not shown).
However, only the output element 57 is typically replaceable in the conventional programmable controller shown in FIG. 19. Any defective elements in the input unit must still be corrected by replacing the whole input unit 53 or swapping a card within the input unit.